This invention relates to a temperature control apparatus and, more particularly, to a temperature control apparatus for controlling the temperature of flat heating equipment such as an electric blanket or an electric carpet.
Electric blankets and electric carpets are safely and extensively used. Such flat heating equipment comprises a combination of a cord like heater and a cord-like thermal sensor. The heater is on-off controlled according to thermal data obtained from a thermal sensor. The heater is turned on and off in synchronism with zero crossings of the AC output of an AC power source. Usually, a thyristor which is connected in series with the heater is used for this. To be more specific, in temperature control the temperature detected by the thermal sensor is compared with a preset temperature and, when the detected temperature is lower than the preset temperature, the heater is supplied with power by turning on the thyristor in synchronism with a zero crossing of the AC power source output. When the detected temperature exceeds the preset temperature, the thyristor is turned off in synchronism with a zero crossing. Consequently, the heater is no longer energized. In this way, the electric blanket or carpet is kept at a preset temperature when it is in use.
The temperature control system of this type includes a safety circuit, which functions to prevent an abnormal temperature increase in the electric blanket or carpet when a fault occurs in the temperature control section. In most cases, the safety circuit can prevent an abnormal increase in temperature. However, if there is a fault in the safety circuit itself, it is not possible to prevent a temperature increase by discontinuing power supply to the heater.